Toejam, at least get that secondary in. If the DO letter doesn't get there for a couple more weeks, getting that secondary in certainly won't hurt you. Who knows, there is always the off chance that they'll accept your MD letter (some schools supposedly will, some absolutely won't).
As far as waiting for a while to turn in secondaries, the only way it will hurt you is when the class starts filling up. Less spots + same amount of applicants = lower chance of acceptance. I highly doubt that they'll look at AACOMAS transmission dates, secondary received dates, and make any decisions that way. And if they were like that, would you want to go there anyway? 🙂
It's easy for many (not saying you are this way, you've been around the block before) to see other people getting interviews and then panic. It's still pretty early in the application cycle. Lots more people will be applying in October, sweating interviews. Hopefully we'll already have our spots. 🙂